Engine fuel systems typically use a fuel filter to remove contaminants from the fuel before the fuel is injected into the engine. After some operating time, these filters can become oversaturated with contaminants and limit the fuel supplied to the engine. During normal operating conditions, enough fuel may be supplied to the engine to operate at a desired air/fuel ratio. However, at certain operating conditions, such as, for example, high speed and high load wide open throttle conditions, there may be a lack of fuel to operate at the desired air/fuel ratio. In this example the desired air/fuel ratio is rich of stoichiometry. During such conditions, there may be a lack of fuel that will degrade engine performance due to excessively lean air/fuel ratio operation.
One method of learning fueling system degradation is to use information provided from closed loop air/fuel ratio control. In such a system, feedback from exhaust gas oxygen sensors located downstream of the engine allow the air/fuel ratio controller to keep the air/fuel ratio at stoichiometry. In addition, a relationship between a fueling command and expected fuel flow rate is known. Thus, during closed loop operation, it is possible to diagnose degradation of the fuel system by comparing the closed loop correction from the exhaust gas sensors to an open loop estimate based on the known relationship. The open loop estimate of fuel flow deviation is provided by an experimentally determined relationship between a pulse width command from the engine controller and mass flow rate of fuel. Such a system is disclosed in U.S. Pat. No. 4,638,658.
The inventors herein have recognized a disadvantage with the above approach. For example, during wide open throttle conditions, which are the only conditions in which to detect certain degradation as described previously herein, the desired air/fuel ratio is enriched and the air/fuel ratio is controlled in an open loop mode. The open loop mode is used because typically exhaust oxygen sensors which switch around stoichiometry are employed. Thus, the closed loop correction value is never learned during this mode of operation and the degradation is not detected.